1. Field of the Invention
The present invention is directed to aqueous dispersions of encapsulated polyisocyanates which have improved pot life and to their use in compositions containing waterborne isocyanate-reactive compounds.
2. Description of the Prior Art
Water dispersible polyisocyanates and processes for achieving water dispersibility of hydrophobic polyisocyanates through chemical modification are known and disclosed in, e.g., U.S. Pat. Nos. 4,176,118, 4,144,267, 4,142,030, 4,097,422, 4,052,347, 3,959,348 and 3,959,329. It is also known to use hydrophobic polyisocyanates in combination with aqueous solutions or suspensions as disclosed in U.S. Pat. Nos. 4,105,594 and 4,129,696. Further, it is known from British Patent 1,186,771 to use external surfactants or additives which are not reactive with isocyanate groups to achieve dispersibility of these hydrophobic isocyanates in water. Aqueous dispersions of hydrophobic polyisocyanates can also be generated in the absence of surfactants or additives using mechanical devices which provide high shear forces.
U.S. Pat. No. 4,904,522 is directed to the preparation of water dispersible polyisocyanates for use in fiberglass binder applications. In addition, U.S. Pat. No. 4,663,377 is directed to the use of water dispersible polyisocyanates in adhesive applications.
The disadvantage of all of these products is that after they are dispersed in aqueous media, they begin to react with water, which means these products have a limited potlife. Waiting too long to apply the product usually results in precipitation and/or formation of insoluble, non-film forming ureas. Even when these products are applied directly after the aqueous dispersions are formed, there is a problem. If film formation occurs before the water/isocyanate reaction is complete, subsequent carbon dioxide formation will result in bubbles in the film rendering it useless for coatings applications. Although undesirable, porous films may still be acceptable as binders for adhesive applications when large surface areas are formed and the resulting polyurethane/urea acts as a spot-type glue between, e.g., fibers, the carbon dixoide gas formation has no negative impact on performance.
One solution for this problem that will lead to non-porous films is to block the isocyanate groups of the water dispersible isocyanate prior to dispersing it in water as described in U.S. Pat. Nos. 4,522,851, 4,098,933 and 4,284,544. The disadvantage of the blocked isocyanate groups is that they require elevated temperatures (above 100.degree. C.) to achieve deblocking and cure and cannot be used as crosslinkers for aqueous systems at room temperature.
Accordingly, an object of the present invention is to provide aqueous polyisocyanate dispersions which have improved pot lives, may be cured at room temperature and exhibit film forming properties.
Surprisingly, this object may be achieved in accordance with the present invention by reacting the dispersed polyisocyanates with polyamines in order to encapsulate them.